1. Field of the Invention
The present invention relates to a water immersion detecting circuit which is provided on a printed board of an electronic unit and is used for detecting water immersion of the electronic unit based on a change of resistance generated between a pair of sensor electrodes for detecting water immersion. More particularly, the present invention relates to a water immersion detecting circuit which is characterized by arrangements of the sensor electrodes, and which is highly resistant to malfunction due to dampness and dew condensation, and further which actuates accurately when the electronic unit is immersed in water.
2. Prior Art
In a related prior art technology, the following circuit is typically used. FIG. 9 is a circuit diagram of the conventional water immersion detecting circuit, in which P10 and P11 designate sensor electrode pads for detecting water immersion, Q10 designates an output transistor for detecting water immersion, R10 designates a resistor for regulating the detection sensitivity, R11 designates an output resistor for detecting water immersion, V10 designates a power supply, and OUT1 designates an output terminal for detecting water immersion. The water immersion detecting circuit shown in FIG. 9 is mounted on a printed board of an electronic unit (not shown). The water immersion detecting circuit is water-proof except for the sensor electrode pads P10, P11.
FIG. 10 shows the structure of the sensor electrodes for detecting water immersion provided in the water immersion detecting circuit of FIG. 9. The sensor electrode pads P10, P11 are placed in the same plane of the printed board 1 provided in the electronic unit in such a way that they are positioned adjacently and oppositely to each other. The sensor electrode pad P10 is connected to the ground.
As mentioned above, in the conventional sensor electrodes for detecting water immersion, two sensor electrode pads P10, P11 are adjacently and oppositely positioned in the same plane of the printed board 1, and when the electronic unit is immersed in water, the opposed sensor electrode pads P10, P11 are electrically connected and the circuit becomes continuous. The continuity of the circuit is detected and current flowing in the circuit is amplified such that the increased current is obtained for detecting water immersion.
Referring to FIGS. 9 and 10, operation of the water immersion detecting circuit will be described.
Generally, river water, sea water and the like water is conductive. When the electronic unit incorporating the water immersion detecting circuit shown in FIG. 9 falls into water such as river water or sea water, a leakage current flows between the sensor electrode pads P10, P11 since the sensor electrode pads P10, P11 are electrically connected through the conductive water and the circuit is made. In this case, the current strength depends on the conductivity of the immersed water, shape of the sensor electrode pads P10, P11, distance between the opposite sensor electrodes, voltage of the power supply and the like. Leakage current flows from the power supply V10 through the resistor R10 when the current value is smaller. However, when the current value is greater than a certain amount, the transistor Q10 is biased and the emitter and the collector of the transistor Q10 are placed in On-state, thereby yielding output for detecting water immersion at the output terminal OUT1 of the resistor R11.
The resistor R10 shown in FIG. 9 is for defining leakage current which urges the transistor Q10. The resistor 10 prevents false detection due to fine leakage current resulting from dampness, dew condensation, noise or the like. The resistor 10 also ensures stable operation of the circuit as well as determines the detection sensitivity of the circuit. The transistor Q10 is not biased unless leakage current flows through the resistor R10 and the potential difference at both ends of the resistor R10 reaches to a certain value for connecting the emitter and the collector of the transistor Q10.
However, in the conventional water immersion detecting circuit, since the sensor electrode pads are mounted on the printed board in such a way that they are adjacent and opposite to each other, false detection would be made when dew covers the electronic unit. This is because when the sensor electrode pads are covered with waterdrops, a leakage current flows between the sensor electrode pads and water immersion is detected.
In order to prevent such false detection and provide a water immersion detecting circuit with suitable sensitivity, highly educated technology and experience are required so as to modify the shape of the sensor electrode pads, structural design of the circuit including a space between the sensor electrode pads, and the amplification degree of the transistor as well as to calculate base resistance and sensitizing current in the circuit.
In view of the above, the present invention seeks to provide a water immersion detecting circuit, which is free from false detection by dew condensation or the like, and is stable and highly sensitive in operation.
According to the present invention, there is provided (1) a water immersion detecting circuit provided on a printed board of an electronic unit and used for detecting water immersion of the electronic unit based on a change of resistance generated between a pair of sensor electrodes for detecting water immersion, wherein the pair of sensor electrodes are positioned opposite to each other on the printed board, and an opening is formed between the sensor electrodes.
There is also provided (2) a water immersion detecting circuit according to the circuit (1), wherein the opening is a U-shaped slit.
There is also provided (3) a water immersion detecting circuit according to the circuit (1), wherein the opening is a V-shaped slit.
There is also provided (4) a water immersion detecting circuit according to the circuit (1), wherein the opening is a hole.
Another object of the present invention is to provide (5) a water immersion detecting circuit provided on a printed board of an electronic unit and used for detecting water immersion of the electronic unit based on a change of resistance generated between a pair of sensor electrodes for detecting water immersion, wherein each of the sensor electrodes is positioned on both front and reverse sides of the printed board respectively.
Further object of the present invention is to provide (6) a water immersion detecting circuit provided on a printed board of an electronic unit and used for detecting water immersion of the electronic unit, wherein a single sensor electrode for detecting water immersion is provided on the printed board of the electronic unit, and water immersion of the electronic unit is detected based on a change of resistance between the sensor electrode and the grounding point of the electronic unit.
Other objects and features of the present invention will become apparent by reference to the following detailed description taken in conjunction with the accompanying drawings.